Cream of tartar, vinegar, and brown sugar are all commonly
used in toffees. Each of these ingredients is acidic,
and with reference to candy-making, each is referred to
as an interfering substance, a term that aptly describes
its function in a boiling syrup.

In
a candy syrup, the sucrose molecules of sugar react
with water in the presence of an acid to form an invert
sugar. This means that as boiling continues, a portion
of the sugar separates into its constituent partsglucose
and fructose. Adding cream of tartar and a dash of vinegar
to a toffee recipe helps bring about this change. The
presence of invert sugar is important because it influences
the amount of crystal formation as well as the size
of the crystals that form as candy hardens.

The bottom line on this one is that the proportion of
interfering substances in the syrup has a strong influence
on the texture of the finished toffee. Candy-making
is tricky, however, because lots of other factors come
into play. Do check out the candy
section for more information.

We suspect that at this time of year your question will
be of interest to other people too. Thanks for asking
it!